/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkEntryNotNull;

import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.concurrent.LazyInit;
import com.google.j2objc.annotations.RetainedWith;

import java.util.function.BiConsumer;
import javax.annotation.CheckForNull;

/**
 * Implementation of {@link ImmutableMap} with exactly one entry.
 *
 * @author Jesse Wilson
 * @author Kevin Bourrillion
 */
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // uses writeReplace(), not default serialization
@ElementTypesAreNonnullByDefault
final class SingletonImmutableBiMap<K, V> extends ImmutableBiMap<K, V>
{

    final transient K singleKey;
    final transient V singleValue;

    SingletonImmutableBiMap(K singleKey, V singleValue)
    {
        checkEntryNotNull(singleKey, singleValue);
        this.singleKey = singleKey;
        this.singleValue = singleValue;
        this.inverse = null;
    }

    private SingletonImmutableBiMap(K singleKey, V singleValue, ImmutableBiMap<V, K> inverse)
    {
        this.singleKey = singleKey;
        this.singleValue = singleValue;
        this.inverse = inverse;
    }

    @Override
    @CheckForNull
    public V get(@CheckForNull Object key)
    {
        return singleKey.equals(key) ? singleValue : null;
    }

    @Override
    public int size()
    {
        return 1;
    }

    @Override
    public void forEach(BiConsumer<? super K, ? super V> action)
    {
        checkNotNull(action).accept(singleKey, singleValue);
    }

    @Override
    public boolean containsKey(@CheckForNull Object key)
    {
        return singleKey.equals(key);
    }

    @Override
    public boolean containsValue(@CheckForNull Object value)
    {
        return singleValue.equals(value);
    }

    @Override
    boolean isPartialView()
    {
        return false;
    }

    @Override
    ImmutableSet<Entry<K, V>> createEntrySet()
    {
        return ImmutableSet.of(Maps.immutableEntry(singleKey, singleValue));
    }

    @Override
    ImmutableSet<K> createKeySet()
    {
        return ImmutableSet.of(singleKey);
    }

    @CheckForNull
    private final transient ImmutableBiMap<V, K> inverse;
    @LazyInit
    @RetainedWith
    @CheckForNull
    private transient ImmutableBiMap<V, K> lazyInverse;

    @Override
    public ImmutableBiMap<V, K> inverse()
    {
        if (inverse != null)
        {
            return inverse;
        }
        else
        {
            // racy single-check idiom
            ImmutableBiMap<V, K> result = lazyInverse;
            if (result == null)
            {
                return lazyInverse = new SingletonImmutableBiMap<>(singleValue, singleKey, this);
            }
            else
            {
                return result;
            }
        }
    }
}
